Editing Stellar nucleosynthesis (section)

==Key reactions==
[[File:Nucleosynthesis periodic table.svg|thumb|right|500px|A version of the periodic table indicating the origins – including stellar nucleosynthesis – of the elements.]]
The most important reactions in stellar nucleosynthesis:
* [[Hydrogen]] fusion:
** [[Deuterium fusion]]
** The [[proton–proton chain]]
** The [[CNO cycle|carbon–nitrogen–oxygen cycle]]
* [[Helium]] fusion:
** The [[triple-alpha process]]
** The [[alpha process]]
* Fusion of heavier elements:
** [[Lithium burning]]: a process found most commonly in [[brown dwarf]]s
** [[Carbon-burning process]]
** [[Neon-burning process]]
** [[Oxygen-burning process]]
** [[Silicon-burning process]]
* Production of elements heavier than [[iron]]:
** [[Neutron]] capture:
*** The [[r-process]]
*** The [[s-process]]
** [[Proton]] capture:
*** The [[rp-process]]
*** The [[p-process]]
** [[Photodisintegration]]

===Hydrogen fusion===
{{Redirect|Hydrogen burning|the combustion of hydrogen gas|Hydrogen#Combustion}}
{{Main|Proton–proton chain reaction|CNO cycle|Deuterium fusion}}
{{multiple image
| align     = right
| direction = vertical
| width     = 300
| image1    = Fusion in the Sun.svg
| caption1  = '''Proton–proton chain reaction'''
| image2    = CNO Cycle.svg
| caption2  = '''CNO-I cycle'''<br />The helium nucleus is released at the top-left step.
}}
Hydrogen fusion (nuclear fusion of four protons to form a [[helium-4]] nucleus<ref name=jones2009/>) is the dominant process that generates energy in the cores of [[main sequence|main-sequence]] stars. It is also called "hydrogen burning", which should not be confused with the [[Chemical reaction|chemical]] [[hydrogen#Combustion|combustion of hydrogen]] in an [[oxidizing]] atmosphere. There are two predominant processes by which stellar hydrogen fusion occurs: [[proton–proton chain]] and the carbon–nitrogen–oxygen (CNO) cycle. Ninety percent of all stars, with the exception of [[white dwarfs]], are fusing hydrogen by these two processes.<ref>Seeds, M. A., ''Foundations of Astronomy'' ([[Belmont, California|Belmont, CA]]: [[Cengage|Wadsworth Publishing Company]], 1986), p. 245.</ref>{{rp|245}}

In the cores of lower-mass main-sequence stars such as the [[Sun]], the dominant energy production process is the [[proton–proton chain reaction]]. This creates a helium-4 nucleus through a sequence of reactions that begin with the fusion of two protons to form a [[deuterium]] nucleus (one proton plus one neutron) along with an ejected positron and neutrino.<ref name=bohm_vitense1992/> In each complete fusion cycle, the proton–proton chain reaction releases about 26.2&nbsp;MeV.<ref name=bohm_vitense1992/> Proton-proton chain with a dependence of approximately T{{sup|4}}, meaning the reaction cycle is highly sensitive to temperature; a 10% rise of temperature would increase energy production by this method by 46%, hence, this hydrogen fusion process can occur in up to a third of the star's radius and occupy half the star's mass. For stars above 35% of the Sun's mass,<ref name=aaa496_3_787/> the [[energy flux]] toward the surface is sufficiently low and energy transfer from the core region remains by [[radiative heat transfer]], rather than by [[Convection (heat transfer)|convective heat transfer]].<ref name=deloore_doom1992/> As a result, there is little mixing of fresh hydrogen into the core or fusion products outward.

In higher-mass stars, the dominant energy production process is the [[CNO cycle]], which is a [[catalytic cycle]] that uses nuclei of carbon, nitrogen and oxygen as intermediaries and in the end produces a helium nucleus as with the proton–proton chain.<ref name=bohm_vitense1992/> During a complete CNO cycle, 25.0&nbsp;MeV of energy is released. The difference in energy production of this cycle, compared to the proton–proton chain reaction, is accounted for by the energy lost through [[neutrino]] emission.<ref name=bohm_vitense1992/> CNO cycle is highly sensitive to temperature, with rates proportional to the 16th to 20th power of the temperature; a 10% increase in temperature would result in a 350% increase in energy production. About 90% of the CNO cycle energy generation occurs within the inner 15% of the star's mass, hence it is strongly concentrated at the core.<ref name=jeffrey2010/> This results in such an intense outward energy flux that [[convective]] energy transfer becomes more important than does [[radiative transfer]]. As a result, the core region becomes a [[convection zone]], which stirs the hydrogen fusion region and keeps it well mixed with the surrounding proton-rich region.<ref name=karttunen_oja2007/> This core convection occurs in stars where the CNO cycle contributes more than 20% of the total energy. As the star ages and the core temperature increases, the region occupied by the convection zone slowly shrinks from 20% of the mass down to the inner 8% of the mass.<ref name=jeffrey2010/> The Sun produces on the order of 1% of its energy from the CNO cycle.<ref>{{Cite web|title=Neutrinos yield first experimental evidence of catalyzed fusion dominant in many stars|url=https://phys.org/news/2020-11-neutrinos-yield-experimental-evidence-catalyzed.html|access-date=2020-11-26|website=phys.org|language=en}}</ref>{{efn|In the [https://www.nature.com/articles/s41586-020-2934-0 November 2020] issue of [[Nature (journal)|''Nature'']], particle physicist Andrea Pocar points out, "Confirmation of CNO burning in our sun, where it operates at only one percent, reinforces our confidence that we understand how stars work."}}<ref>[[Gregory Robert Choppin|Choppin, G. R.]], [[Jan-Olov Liljenzin|Liljenzin, J.-O.]], [[Jan Rydberg|Rydberg, J.]], & [[:sv:Christian Ekberg|Ekberg, C.]], ''Radiochemistry and Nuclear Chemistry'' (Cambridge, MA: [[Academic Press]], 2013), [https://books.google.com/books?id=CN88gBPtiucC&pg=PA357&redir_esc=y#v=onepage&q&f=false p. 357].</ref>{{rp|357}}<ref>{{Cite journal|last1=Agostini|first1=M.|last2=Altenmüller|first2=K.|last3=Appel|first3=S.|last4=Atroshchenko|first4=V.|last5=Bagdasarian|first5=Z.|last6=Basilico|first6=D.|last7=Bellini|first7=G.|last8=Benziger|first8=J.|last9=Biondi|first9=R.|last10=Bravo|first10=D.|last11=Caccianiga|first11=B.|date=25 November 2020|title=Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun|url=https://www.nature.com/articles/s41586-020-2934-0|journal=Nature|language=en|volume=587|issue=7835|pages=577–582|doi=10.1038/s41586-020-2934-0|pmid=33239797|issn=1476-4687|arxiv=2006.15115|bibcode=2020Natur.587..577B|s2cid=227174644}}</ref>{{efn|"This result therefore paves the way toward a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent."—M. Agostini, et al.}}

The type of hydrogen fusion process that dominates in a star is determined by the temperature dependency differences between the two reactions. The proton–proton chain reaction starts at temperatures about {{val|4|e=6|ul=K}},<ref name=reid_hawley2005/> making it the dominant fusion mechanism in smaller stars. A self-maintaining CNO chain requires a higher temperature of approximately {{val|1.6|e=7|u=K}}, but thereafter it increases more rapidly in efficiency as the temperature rises, than does the proton–proton reaction.<ref name=salaris_cassini2005/> Above approximately {{val|1.7|e=7|u=K}}, the CNO cycle becomes the dominant source of energy. This temperature is achieved in the cores of main-sequence stars with at least 1.3 times the mass of the [[Sun]].<ref name=apj701_1_837/> The Sun itself has a core temperature of about {{val|1.57|e=7|u=K}}.<ref>Wolf, E. L., ''Physics and Technology of Sustainable Energy'' ([[Oxford]], [[Oxford University Press]], 2018), [https://books.google.com/books?id=BP9eDwAAQBAJ&pg=PA5&redir_esc=y#v=onepage&q&f=false p. 5].</ref>{{rp|5}} As a main-sequence star ages, the core temperature will rise, resulting in a steadily increasing contribution from its CNO cycle.<ref name=jeffrey2010/>

===Helium fusion===
{{Main|Triple-alpha process|Alpha process}}
Main sequence stars accumulate helium in their cores as a result of hydrogen fusion, but the core does not become hot enough to initiate helium fusion.  Helium fusion first begins when a star leaves the [[red giant branch]] after accumulating sufficient helium in its core to ignite it. In stars around the mass of the Sun, this begins at the tip of the red giant branch with a [[helium flash]] from a [[Degenerate matter|degenerate]] helium core, and the star moves to the [[horizontal branch]] where it burns helium in its core. More massive stars ignite helium in their core without a flash and execute a [[blue loop]] before reaching the [[asymptotic giant branch]]. Such a star initially moves away from the AGB toward bluer colours, then loops back again to what is called the [[Hayashi track]]. An important consequence of blue loops is that they give rise to classical [[Cepheid variable]]s, of central importance in determining distances in the [[Milky Way]] and to nearby galaxies.<ref>Karttunen, H., Kröger, P., Oja, H., Poutanen, M., & Donner, K. J., eds., ''Fundamental Astronomy'' ([[Berlin]]/[[Heidelberg]]: [[Springer Science+Business Media|Springer]], 1987), [https://books.google.com/books?id=DjeVdb0sLEAC&pg=PA250&redir_esc=y#v=onepage&q&f=false p. 250].</ref>{{rp|250}} Despite the name, stars on a blue loop from the red giant branch are typically not blue in colour but are rather yellow giants, possibly Cepheid variables. They fuse helium until the core is largely [[carbon]] and [[oxygen]]. The most massive stars become supergiants when they leave the main sequence and quickly start helium fusion as they become [[red supergiant]]s. After the helium is exhausted in the core of a star, helium fusion will continue in a shell around the carbon–oxygen core.<ref name=jones2009/><ref name=deloore_doom1992/>

In all cases, helium is fused to carbon via the triple-alpha process, i.e., three helium nuclei are transformed into carbon via [[Beryllium-8|<sup>8</sup>Be]].<ref>Rehder, D., ''Chemistry in Space: From Interstellar Matter to the Origin of Life'' ([[Weinheim]]: [[Wiley-VCH]], 2010), [https://books.google.com/books?id=baI91e8lgm0C&pg=PT30&redir_esc=y#v=onepage&q&f=false p. 30].</ref>{{rp|30}} This can then form oxygen, neon, and heavier elements via the alpha process. In this way, the alpha process preferentially produces elements with even numbers of protons by the capture of helium nuclei. Elements with odd numbers of protons are formed by other fusion pathways.<ref>[[Michael Perryman|Perryman, M.]], ''The Exoplanet Handbook'' (Cambridge: Cambridge University Press, 2011), [https://books.google.com/books?id=ngtmDwAAQBAJ&pg=PA398&redir_esc=y#v=onepage&q&f=false p. 398].</ref>{{rp|398}}